1. Field of the Invention
The present invention relates to novel N-vinyllactam derivatives and polymers thereof for use in microlithography. More particularly, the present invention relates to N-vinyllactam derivatives and polymers thereof, used as materials for photoresist which is capable of forming picture of high sensitivity and high resolution by use of deep UV and to homo- and copolymers thereof for use as photoresist.
2. Description of the Prior Art
Usually, photoresist consists mainly of an alkali-soluble phenol-(or cresol-)formaldehyde novolak resin and a substituted naphthoquinone diazide compound as a photosensitive material (photoactive ingredient), as described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470.
While the novolak resin used in such photoresist is dissolved in an aqueous alkali solution, the naphthoquinone photosensitive material acts as a dissolution inhibitor of resist. However, when a substrate coated with the photoresist is selectively subjected to chemical radiation, the photosensitive agent is induced to suffer from such structural modification that the exposed region of the photoresist coating is of stronger solubility to alkali than the unexposed region. By virtue of such differences in solubility, a relief pattern can be carved on the coating of the substrate. That is, when the substrate is immersed in an alkaline developing solution, the exposed region of the photoresist coating is dissolved whereas the unexposed region is not substantially affected, forming a pattern. However, the above-mentioned novolak type resists were not found to be suitable to the steper utilizing shorter wavelength, which will be used in the future, because they show high optical absorbance in the range of deep ultraviolet light, 200 to 300 nm.
In order to accomplish high sensitivity in the lithography process of semiconductor manufacture, chemical amplification resist has recently been developed. Indeed, the chemical amplification resist has been in the limelight since it was found to have the capacity for increasing sensitivity 100-fold over conventional positive novolak resists. Chemical amplification resist, which takes advantage of the photoacid generator (hereinafter referred to as "PAG"), is generally prepared by formulating PAG in a matrix polymer of a structure sensitively reacting to acid. For the mechanism of the photoreaction, when PAG is exposed to light or irradiated by a high energy beam, such as X-ray and electron beam, strong protonic acid, Bronsted acid, are generated, causing the main chain or the side chain of the matrix polymer to react toward decomposition, crosslinking or large change in polarity. This action of the acid induces, at the irradiated region, the solubility of the given developing solution to be altered. That is, increased or decreased. As a result, fine patterns can be formed.
Onium salt which is able to respond to light and radiation is known as the photoacid generator. Onium Salt typically includes ammonium salts, oxonium salts and sulfonium salts, etc. Recently, it has been reported that organic sulfonic ester can function as the photoacid generated.
Available for the matrix polymer, which can react with acid, is for example, polymers having a side chain such as t-butylester, t-butylcarbonate, t-butoxy or t-butoxycarbonyl groups, which can be decomposed into carboxylic acid, phenol or alcoholic functional group by acid. Among such side chain protecting groups, the t-butoxycarbonyl group is highest in sensitivity. Such acid-reactable polymer in a protected state or prior to reaction with acid, can be dissolved in an organic solvent but insoluble in an alkali aqueous solution. However, if the acid-reactable polymer is deprotected by reaction with acid, it is soluble in alkali aqueous solution because its polarity is significantly changed.
Using this principle, the development of chemical amplification resists has been a hot issue in recent years. T-Butoxycarbonyl-protected polyvinylphenol (hereinafter referred to as "t-bocPVP") is reported to be one of the most promising resins, as introduced in U.S. Pat. Nos. 4,491,628, 4,405,708 and 4,311,782.
A recent trend in submicrolithography is to use as a light source deep uv (wavelength 200 to 300 nm), preferably, a KrF excimer laser of high power (wavelength 248), rather than conventional uv, e.g. g-line (wavelength 436 nm) or i-line (wavelength 365 nm), in order to accomplish high sensitivity and high resolution. Therefore, the optical absorption of the matrix polymer should be minimized in the wavelength range of deep uv, particularly at 248 nm, the wavelength of KrF excimer laser. However, since t-bocPVP also contains the benzene group, it has the significant disadvantage of showing large optical absorption in short wavelength ranges.